Circuitry for pull-in solenoids



Aug. 14, 1962 R. c. GREENBLATT 3,049,650

CIRCUITRY FOR PULL-IN SOLENOID-S Filed April 15, 1959 3 Sheets-Sheet 1+E,(+6v) E2(-I8V) +E3(+|.5V) E4(-22V) 0 (A) INPUT PULSE (B) COLLECTORVOLTAGE OF T.

(C) BASE VOLTAGE OF T 0 -v F192 8D (0) BASE cURRENT T2.

(5) COIL CURRENT (F) COLLECTOR VOLTAGE OF T2.

k JNVENTOR.

ATTORNEY Aug. 14, 1962 R. c. GREENBLATT 3,049,650

CIRCUITRY FOR PULL-IN SOLENOIDS Filed April 15. 1959 3 Sheets-Sheet 2 T10 r1 r1 |2 COLLECTOR VOLTAGE (A) OF T I -Diode Reslsfunce Ciump SpikeCOLLECTOR 9 5 3 CURRENT L OF T --Diode Resistance Clamp O U rt SOLENOIDCOIL 5 CURRENT (C) Dxode Resistance Clamp s ,t J

3 t t r t 0 E S g H F Due T0 T Alone 4 INVENTOR; 8 Du To RICHARD c.GREENBLATT E5 Al ;?\e- BY 4/ 5 y M ATTORNEY 1962 R. c. GREENBLATT3,049,650

CIRCUITRY FOR PULL-IN SOLENOIDS Filed April 15, 1959 3 Sheets-Sheet 5COLLECTOR VOLTAGE OF T2.

E V rt 5 3 COLLECTOR CURRENT L (B) TL F lg. 7 Q

E COIL CURRENT L5 .5 x U K/ Vt INVEN TOR.

RICHARD C. GREENBLATT ATTORNEY United States 3,049,650 CIRCUITRY FORPULL lN SQLENOHDS Richard C. Greenhlatt, Malvern, Pa, assignor toBurroughs Corporation, Detroit, Mich, a corporation of Michigan FiledApr. 15, 1959, Ser. No. 806,649 4 Claims. (Cl. 317148.5)

miniature solenoids in present day use are required to develop a pullequal to or greater than that of their structurally larger predecessors.In addition to these consider ations, it is also necessary that theminiature solenoid coil and its supporting circuitry be capable ofresponding to input pulse signals of varying time widths and to dutycycles which are of considerable duration. For reliable operation, theminiature solenoid must be capable of both one-shot and sustainedhold-in operation under the worse anticipated conditions without asignificant increase in coil temperature. pable of operation underconditions which are equivalent to changes in coil supply voltage ashigh as 50% above and 50% below nominal. These wide supply voltageexcursions are, of course, not experienced in practice, but in designthese voltage magnitudes are used to simulate the effects of agingand/or wear of the mechanical components, in order to insure long termreliability of the overall system.

The solenoid coil itself presents an inductive load to its driver.driver is a transistor operating with such an inductive load, theproblem of backswing arises, the so called backswing being evidenced bya high voltage spike waveform which is a function of the magnitude ofthe instantaneous transistor current at out off. These voltage spikes donot cause instantaneous failure, but their effects are cumulative, andafter a period of time, the transistor fails.

In some instances, it may be possible to mitigate the situation byfavorably changing the causative factors.

However, in the overwhelming bulk of practical problems encountered,these environmental conditions are immutable and one must operate withinthe metes and bounds which they delineate.

In accordance with a preferred embodiment, there is provided a circuitfor utilization in operating a pull-in solenoid. A transistor having abase, an emitter and a collector is arranged in common emitterconfiguration. Means are used for connecting the solenoid coil betweenthe collector and a source of biasing potential. Means are arranged forconnecting one end of each of first and second resistors to biasingpotential sources respectively, a

condenser being serially connected respectively between the other endsof these resistors. The connection of the first resistor with thecondenser defines a junction point. The connection of the secondresistor with the condenser is also common to the base of thetransistor, and means are arranged for alternately changing thepotential of the junction point from ground to a predetermined negativepotential.

In accordance with another preferred embodiment, there is providedadditional circuitry cooperatively connected Finally, the solenoid coilmust be ca- It is a well known phenomenon that where the atent ice withthe embodiment previously described. This circuitry includes a secondtransistor having a base, a collector and an emitter, the lattercollector being connected to the collector of said first transistor.Means are provided for connecting the emitter of the second transistorto a third resistor in series with a source of biasing potential. Asource of DC. potential having a magnitude greater than that of saidlatter biasing potential is also utilized. A diode having a cathode andan anode has its cathode connected to the base of the second transistorand to the DC. potential source, the anode thereof being returned toground. Finally a current limiting resistor is arranged in a closed loopconnecting the base of the second transister with the junction point.

Accordingly, it is an object of this invention to provide an improvedtransistor driver for a pull-in solenoid which is capable of operatingunder a variety of adverse environmental conditions.

A further object is to provide an improved transistor driver of reliableoperation which consumes a minimum of power and is of low-cost tomanufacture.

The novel features which are believed to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconnection with the accompanying drawings in which:

FIG. 1 is a circuit diagram of an improved transistor driver inaccordance with the invention, the driver being for use in one-shotoperation;

FIG. 2 includes a number of voltage and current waveforms used inexplaining the operation of FIG. 1;

FIG. 3 is a circuit diagram of an improved transistor driver for apull-in solenoid for utilization in sustained holding operation;

FIG. 4 includes a number of voltage and current waveforms used toexplain the operation of FIG. 3;

FIG. 5 includes a number of voltage and current waveforms used toexplain the problem posed by backswing in driver operation;

FIG. 6 is a circuit diagram of an improved driver for a pull-in solenoidutilized to solve the problem of backswing when the input signals are ofcertain time magnitudes; and

FIG. 7 includes a series of voltage and current waveforms used toexplain the operation of FIG. 6.

Referring now to FIG. 1 of the drawings, two transistors T and T areconnected in the common emitter con figuration. The input signal isapplied via terminal 10 and ground, through resistor 12 to the base oftransistor T Bias potentials +13 E for the base and collector oftransistor T are applied through resistors 14 and 16 respectively. Thetransistor T is coupled to transistor T by means of a capacitor 18.

Bias potentials for the base and collector of transistor T are indicatedat +E and E respectively, the battery +13 being connected to the basethrough resistance 20, while battery E., is connected to the collectorof T through the solenoid coil 22.

The transistors in the illustrative embodiment are of the PNP type;however, it is within the scope of this invention to utilize NPN typesif proper attention is given to the biasing potentials and triggeringsignals.

At this point it will be helpful to briefly discuss the operation of thecircuit of FIG. 1 and for this purpose reference will be had to FIG. 2.

The input signal pulse shown in FIG. 2A is positive going during theinterval t to t As we shall see as this description proceeds, somedesired objective is accomplished during this interval, andappropriately enough, the time ratio of this period to the entire periodof signal duration is called the duty cycle. The circuitry of theinvention is intended for operation in an environment which may requirea duty cycle in the order of 50% or higher. These environmental desirataare dictated by the fact that the input signal may not be specificallytailored to trigger the solenoid driver of FIG. 1; instead this inputsignal may drive other circuitry in an overall system in which thesolenoid driver of FIG. 1 is only a part.

In the interval -t the transistor amplifier T is ON, and the transistordriver T is cut off. The base of transistor T is at its negativeconducting potential (in order .2 v.), while the collector thereof issubstantially at ground. The base of transistor driver T is at apositive potential with respect to its emitter; this voltage isindicated at +V in FIG. 2C, and it is the result of the biasing voltage+E acting through resistor 20. The notation +V and V are merely used asconvenient means for voltage base up and voltage base down respectively.The collector voltage of transistor T is at E so that it is cut oil.

At time t the positive going pulse depicted in FIG. 2(A) is applied tothe base of T driving it more positive until it is substantially atground or slightly positive. The transistor T cuts off and its collectorreaches a potential of -E This cutting 01f of transistor T is efiectiveto cause transistor T to saturate. The effect of cutting off T isequivalent to the sudden application of a step voltage of magnitude -Eto an RC circuit which may be traced: from ground through the emitterand base of T through condenser 18, resistor 16, -E battery and returnto ground. This charging base current rises to its maximum and thenbegins to decay exponentially. The initial high base current of T FIG.2(D), is of suflicient magnitude to turn it ON and drive it well intosaturation. In the collector circuit of T the opposite conditionprevails because of the high impedance of coil 22. The coil current asshown in FIG. 2(E) builds up in magnitude, until the time t is reached,at which time the clapper or armature of the solenoid pulls in.

The slight cusp in coil current at t may be explained as follows. At atime slightly greater than t the current in the solenoid coil 22 beginsto increase. There are a number of factors which determine the shape ofthe coil current, the role played by each factor varying in importancedepending on the instant of time then under scrutiny. The inductiveproperties of the coil are a function of both the current level and theinstantaneous rate of change of current passing through it. This is sobecause the electromagnetic circuit of the coil includes an air gapwhich is rapidly changed at a given coil current level, and because thearmature and spring system have inertial and restoring propertiesrespectively.

The armature starts to close during the interval t -t this displacementresults in a rapid increase in the magnetic flux density in the core andhence a back voltage is developed to oppose the build up of coil currentin ac- .cordance with the relationship:

where eqhe back voltage N=the number of turns of the solenoid coil =themagnetic flux lines linking the core.

As is well known, the negative sign indicates that when dgb/dl ispostive, e is negative and tends to oppose the voltage causing theincrease in current.

The advance of the armature toward the closed position takes place inincremental steps of increasing magnitude, the final steps beingsufiiciently large to cause a 4 change in flux linkage which develops aback voltage of sufiicient magnitude to cause an instantaneous backvoltage which momentarily reduces the coil current to produce the cusp.

At t the armature is fully closed and the magnetic flux linking the coilis a maximum and is constant, and the back voltage is momentarily zero.The full applied voltage is now applied to the coil. With the armaturein closed position the effective inductance (L) is increased, andcurrent buildup continues toward the asymptotic value determined by themagnitude of -E and the series resistance of coil 22.

The exponential decay of the base current of transistor T FIG. 2(D) isintended to provide a minimum collector current which is equal to thepull-in current required at the longest actual pull-in time of thesolenoid under the worse environmental conditions.

At the instant just prior to t the base of transistor T is at +V as aresult of biasing voltage +E acting through resistor 20. The cut off ofT is equivalent to the application of a step voltage of magnitude |-Ethrough resistor 16, condenser 18, to the base of transistor T Thus asmay be seen in FIG. 2(C) the base to emitter voltage is driven in anegative direction, by an amount determined by the individual transistorcharacteristics to -V The base current of transistor T rises to itsmaximum instantaneously in the manner shown in FIG. 2(D); this currentnow begins to decay exponentially, and as it does the voltage begins toincrease exponentially, and it will approach Zero at time 1 The basevoltage is now at ground, and transistor T is cut off. However, the baseto emitter voltage continues to increase in positive direction as shownin FIG. 2(C); this is due to the fact that capacitor 18 continues tocharge by virue of battery +15 acting through resistor 20.

At time 1 the capacitor 18 is almost fully charged. Upon the terminationof the input pulse, the base voltage of T goes to V this is a negativegoing pulse. Since the transistor T is operated in the common-emitterconfiguration, the negative going pulse at the base of T results in apositive going step pulse at the collector of T Since the charge oncondenser 18 cannot change instantly, a positive going step pulse istransmitted to the base of transistor T which adds to the positivevoltage already present just prior to t The condenser 18 then dischargesto the base voltage of T decaying exponentially toward |V as indicatedat FIG. 2(C).

In the illustrative embodiment just described, the components thereinutilized had the following magnitudes:

Resistor 12:4.5K ohms Resistor 14:20.5K ohms Resistor 16:300 ohmsResistor 20:200 ohms Condenser 18:15 ,ufarad Coil 22:1000 turns #31 wireR -20 ohms L-20 millihem'ys Input 3-7 v. to ground The transistor T maybe a 2N527, while transistor T may be either a 2Nll38A or a 2N285A, bothof which are of the PNP type.

The circuit just described has utility where the armature need not beheld closed for the full duration of the input cycle (t to taccordingly, the coil current is zero at t because its mission isaccomplished, and there is no further need for the coil to carrycurrent. If it is necessary to hold the armature closed for the entirewidth of the input cycle, holding current must be supplied. This may bedone most advantageously by the arrangement shown in FIG. 3.

In the circuit of FIG. 3, similar components are given the samenumerical designation as in FIG. 1; this arrangement includes a thirdtransistor T having its collector connected to the collector of Tthrough a resistance 24. The emitter of transistor T is connected to asource of positive potential +E5 through a resistor 26. The base oftransistor T is connected to the collector of transistor T throughresistor 28 so as to form a closed loop. A source of positive potential+E6 is connected to the base of T 3 through resistor 30. A diode 32 hasits cathode connected to the base of transistor T while the anodethereof is returned to ground.

The transistor T is a constant current driver added for the purpose ofsupplying constant holding current to the solenoid coil 22. Themechanism for accomplishing this is as follows:

When transistor T is con-ducting and transistor T is cut off, thetransistor T is held off by reason of the fact that the base is slightlypositive with respect to the emitter because of the biasing potential+E6 acting through resistor 30. It would also be possible to keep T cutoff without using resistor 30, by making |E |E in the order of magnitudeof 1 volt or so. The biasing voltage E is dropped across resistors 30and 28 in series; this circuit may be traced from ground, battery +Eresistors 30 and 28 in series, and through transistor T (which isconducting) and return to ground. At this point, when the positive goingpulse is applied to the input, the transistors T and T respond exactlyas described in connection with the description of FIG. 1. As T is cutoil by the positive going input pulse, the collector voltage of T decaysexponentially toward the values E and this voltage change is applied tothe base of transistor T through resistor 28. Electrically the potentialof the base of transistor T is also the potential of the cathode ofdiode 32, and as soon as the cathode becomes sufficiently negative, thediode conducts, clamping the base of T substantially at ground. Thetransistor T is now effectively operated in the grounded baseconfiguration. This means that conditions for conduction (since T is ofthe PNP type) require that the emitter and collector be positive andnegative respectively with reference to the base.

At the time indicated as t in FIG. 2, the collector voltage of T beginsto decay toward the level E The collector of transistor T becomes moreand more negative with respect to its own base, as the collector voltageof T- FIG. 2(F) decays toward the E level. T now conducts in a circuitwhich may be traced: Ground, battery +E through resistor 26, through theemitter, base, and collector of transistor T through resistor 24,through solenoid coil 22, battery E and return to ground.

The transistor T is a constant current driver. The collector current i,is approximately equal to the emitter current i and 1, +E5 c e Re whereAs long as the transistor T continues to conduct, the collector currenti will remain substantially constant at this value. Hence the circuitwill continue to provide a constant holding current for the fullduration of the input pulse independent of the current gain variationsof transistor T The solenoid current and the input pulse signal aredepicted in FIG. 4. The individual contributions of transistors T and Tto the total solenoid current are indicated in this latter figure; bysuperposition principles the heavy black line indicates the sum or totalsolenoid current as a function of time. Because of the fact that thearmature is in the closed position, the current necessary to hold it inis of smaller magnitude than that re quired for the initial pull-in.

In the circuit of FIG. 3 the transistor T may be a PNP transistor 2N527and the diode 32 may be a T6G.

In those situations where the input pulse width is of the order of timemagnitude slightly greater than the maximum pull-in time of thesolenoid, i.e., the duty cycle is of an order only slightly greater than(t t then excessive backswing of the collector voltage of transistor Twill result. This backswing is depicted in FIG. 5(A) as a high negativevoltage spike. It is well known that the backswing of a transistor withan inductive load is proportional to the magnitude of the instantaneouscurrent at out off. The transistor does not fail instantly but thedamage is cumulative so that it deteriorates gradually with time. Thetheory offered in explanation of this phenomena is that these spikescause a localized destructive heating of the junction.

A conventional means of reducing the backswing is to use adiode-resistance clamp in parallel with the solenoid coil. Thisarrangement is efficacious in reducing the backswing; however, thiscircuitry has the disadvantage that a much longer time is required forthe coil current to decay to Zero, after the transistor is cut off. Therecovery time is also a function of the magnitude of the coil current atout off. The resulting waveforms of the collector voltage, collectorcurrent and the coil current are shown in dotted form in FIG. 5 (A), (B)and (C) respectively. Such a solution may provide tolerable results inrelatively l-ow duty cycles. However, where recovery time problems ariseas in the case with large duty cycles, the arrangement shown in FIG. 6is superior.

In FIG. 6 there is shown the same circuitry as in FIG. 1, with theexception that a resistor 34 is connected across the solenoid coil 22,and a condenser 36 is connected between the collector and emitter oftransistor T In the DC. sense, resistor 34 and coil 22 are in parallel,this parallel combination being in series with the condenser 36.

The magnitudes chosen for the resistor 34 and the capacitor 36 willdetermine whether the circuit is under damped, critically damped, orover damped, the particular choice depending upon the magnitude ofbackswing that can be tolerated. In one practical embodiment, resistor34 was of the order of 200 ohms and capacitor 36 was in the order of 10afarad.

The relatively large capacitance present in the collector circuit doesnot affect the rise time of the coil current appreciably, since theinitial base current is large enough to permit the condenser 36 todischarge through the transistor T faster than the coil current canrise. In FIG. 7(A), (B), (C) there is shown the collector voltage of Tthe current of T and the coil current respectively, which results areobtained using the damping technique of FIG. 6.

The values and/ or types of components and the voltages appearing on thedrawings are included by way of example only, as being suitable for thedevice illustrated. It is to be understood that the circuitspecifications in accordance with the invention may vary with the designfor any par-ticular application.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced other than as specifically described and illustrated.

What is claimed is:

l. A circuit of the type described for utilization in operating apull-in solenoid relay comprising, first, second and third transistors,each having a base, an emitter and a collector, the first and secondtransistors being arranged with the emitters at ground potential, meansfor connecting the solenoid coil between the collector of the firsttransistor and a source of biasing potential, first and secondresistors, means for connecting one end of each of said first and secondresistors to biasing potential sources respecitvely, a condenserserially connected between the other end of each resistor respectively,the connection of said first resistor with said condenser defining ajunction point, the connection of said second resistor with saidcondenser being electrically common with the base of said firsttransistor, means for connecting the collector of said second transistorwith said junction point, means for connecting the base of said secondtransistor to a source of biasing potential, input means for applyingsignals to the base of said second transistor, the col ector of thethird transistor being connected to the collector of said firsttransistor, a third resistor, means for connecting the emitter of saidthird transistor to said third resistor in series With a source ofbiasing potential, a source of DC. potential having a magnitude greaterthan that of said latter biasing potential, a diode having an anode anda cathode, the cathode of said diode being connected to the base of saidthird transistor and to said DC. potential source, the anode beingconnected to ground, and a current limiting resistor in a closed loopconnecting the base of said third transistor with said junction point.

2. A circuit of the type described for utilization in operating apull-in solenoid relay comprising, a first transistor having a base, anemitter and a collector arranged With the emitter at ground potential,means for connecting the solenoid coil between the collector of saidfirst transistor and a source of biasing potential, first and secondresistors, means for connecting one end of each of said first and secondresistors to biasing potential sources respectively, a

condenser serially connected between the other ends of each of saidresistors respectively, the connection of said first resistor with saidcondenser defining a junction point, the connection of said secondresistor with said condenser being also connected to the base of saidfirst transistor,

means for alternately changing the potential of said junction point fromground to a predetermined negative potential, a second transistor havinga base, a collector and an emitter, the latter collector being connectedto the collector of said first transistor, a third resistor, means forconnecting the emitter of said second transistor to said third resistorin series with a source of biasing potential, a source of DC. potentialhaving a magnitude greater than that of said latter biasing potentialsource, a diode having an anode and a cathode, the cathode of said diodebeing connected to the base of said second transistor and to said D.-C.potential source, the anode being connected to ground, and a currentlimiting resistor in a closed loop connecting the base of said secondtransistor with said junction point.

3. A circuit of the type described for utilization in operating apull-in solenoid relay comprising, a transistor having a base, anemitter and a collector arranged with the emitter at ground potential,means for connecting the solenoid coil between said collector and asource of biasing potential, first, second and third resistors, meansfor connecting one end of each of said first and second resistors tobiasing potential sources respectively, a first condenser seriallyconnected between the other ends of each of said first and secondresistors respectively, the connection of said first resistor with saidfirst condenser defining a junction point, the connection of said secondresistor with said first condenser being also connected to said base,means for alternately changing the potential of said junction point fromground to a predetermined negative potential, a second condenser beingconnected between said collector and ground potential, the thirdresistor being serially connected with said second condenser andarranged in shunt with the solenoid coil.

4. A circuit of the type described for utilization in operating apull-in solenoid relay comprising, first and second transistors, eachhaving a base, an emitter, and a collector and arranged with the emitterat ground potential, means for connecting the solenoid coil between thecollector of said first transistor and a source of biasing potential,first, second and third resistors, means for connecting one end of eachof said first and second resistors to biasing potential sourcesrespectively, a first condenser serially connected between the other endof each of said first and second resistors respectively, the connectionof said first resistor with said first condenser defining a junctionpoint, the connection of said second resistor with said first condenserbeing electrically common with the base of said first transistor, meansfor connecting the collector of said second transistor with saidjunction point, means for connecting the base and collector of saidsecond transistor to sources of biasing potential respectively, a secondcondenser connected between the collector of said first transistor andground, the third resistor being serially connected with said secondcondenser and arranged in shunt with the solenoid coil, and input meansfor applying signals to the base of said second transistor.

References Cited in the file of this patent UNITED STATES PATENTS2,843,762 Trent July 15, 1958 2,896,130 Tompkins July 21, 1959 2,901,639Woll Aug. 25, 1959

